Accounting for Certain Loans Acquired in a Transfer - Amounts of Loans (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	$ 1,749,131	$ 1,378,554
Carrying amount, net of allowance	1,730,806	1,360,971
Loans Purchased With Evidence Of Credit Deterioration [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	10,390	15,626
Carrying amount, net of allowance	10,327	15,267
Loans Purchased With Evidence Of Credit Deterioration [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	8,261	13,217
Loans Purchased With Evidence Of Credit Deterioration [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	2,088	2,358
Loans Purchased With Evidence Of Credit Deterioration [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	41	51
Loans Purchased With Evidence Of Credit Deterioration [Member] | Heartland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	2,332	6,400
Loans Purchased With Evidence Of Credit Deterioration [Member] | Heartland [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	1,633	5,492
Loans Purchased With Evidence Of Credit Deterioration [Member] | Heartland [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	693	900
Loans Purchased With Evidence Of Credit Deterioration [Member] | Heartland [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	6	8
Loans Purchased With Evidence Of Credit Deterioration [Member] | Summit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	6,818	9,226
Loans Purchased With Evidence Of Credit Deterioration [Member] | Summit [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	5,567	7,725
Loans Purchased With Evidence Of Credit Deterioration [Member] | Summit [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	1,216	1,458
Loans Purchased With Evidence Of Credit Deterioration [Member] | Summit [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	35	$ 43
Loans Purchased With Evidence Of Credit Deterioration [Member] | Peoples Bancorp Inc [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	1,240
Loans Purchased With Evidence Of Credit Deterioration [Member] | Peoples Bancorp Inc [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	1,061
Loans Purchased With Evidence Of Credit Deterioration [Member] | Peoples Bancorp Inc [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	$ 179